1. Field of the Invention
This invention relates to current ramping controller circuits and more particularly, to a current ramping controller circuit for charging and discharging a capacitor by accurately matched currents.
2. Description of the Prior Art
There are many contemporary applications requiring ramping of capacitors and therefore the need for a ramping controller circuit for charging and discharging the capacitor. For instance, in most if not all of the electronic ignition systems available today at least one capacitor is utilized which is both charged and discharged accordingly to produce the spark signal which fires the spark plugs in correct time with the operation of the automobile engine. In such applications there is usually the need for generating both charge up and charge down currents for ramping the capacitor up and down wherein both these currents are of the same magnitude to minimize errors in the ignition system.
The method most recognized by the prior art for providing charge up and charge down currents uses a well known current mirror circuit wherein an input current is mirrored to an output. Thus, a PNP transistor current mirror circuit is utilized for providing a sourcing current which is a function of an input current provided thereto for charging the capacitor. Likewise, a NPN current mirror circuit is used for sinking current for discharging the capacitor wherein the sinking current is a function of an input current to the NPN current mirror circuit. A problem arises due to the capability of today's technology in matching the output current of the current mirrors to the input current supplied thereto. Typically, the best that these currents can be matched using contemporary integrated circuit techniques results in a 3% error between the two currents. Thus, in a circuit requiring a PNP current source and NPN current source as aforementioned, there is a possibility of having a maximum 6% error in the magnitudes of the currents which charge up and charge down the capacitor. In some applications this 6% error cannot be tolerated.
Thus, there exists a need for providing a current ramping controller circuit for providing charge and discharging currents for ramping a reference capacitor up and down where the two currents are accurately matched in value with respect to one another.